Metformin is the first-choice medicine for type 2 diabetes. It lowers blood sugar mainly by reducing the amount of glucose the liver releases, and it does not cause weight gain or, on its own, low blood sugar. It comes as 500 mg and 850 mg tablets and in slow-release forms.
What is Metformin?
Metformin hydrochloride is Schedule 3 in South Africa and appears under many names — Glucophage, Glucophage XR and numerous generics:
| Form | Typical strengths |
|---|---|
| Standard tablet | 500 mg, 850 mg, 1000 mg |
| Slow-release (XR/SR) | 500 mg, 750 mg, 1000 mg |
Being dispensed one brand rather than another is ordinary generic substitution.
Metformin is a biguanide, and it works quite differently from most diabetes medicines. It does not make the pancreas produce more insulin. Instead it:
- Reduces glucose production by the liver — its main action. In type 2 diabetes the liver releases glucose even when it is not needed, particularly overnight, which is why morning readings are often high.
- Improves insulin sensitivity, so muscle and fat respond better to the insulin already present.
- Slightly reduces glucose absorption from the gut.
Because it does not force insulin release, metformin does not usually cause hypoglycaemia on its own. This is a genuine advantage: it lowers glucose that is too high without pushing it too low. Hypos become a risk only when metformin is combined with insulin or a sulfonylurea such as gliclazide.
It also does not cause weight gain, and often causes modest weight loss — another reason it is first choice, since many diabetes treatments do the opposite.
Standard and slow-release forms matter practically. The slow-release form releases metformin gradually and causes considerably less stomach upset. If ordinary metformin is upsetting your stomach, that is worth raising — switching to the slow-release form solves it for many people, and stomach upset is a common reason people quietly stop taking it.
What is Metformin commonly used for?
- Type 2 diabetes — the standard first-line treatment, usually started at diagnosis alongside diet and activity changes.
- Prediabetes, in some people at high risk of progressing to diabetes.
- Polycystic ovary syndrome (PCOS), where insulin resistance drives irregular periods and other symptoms. Metformin can help restore ovulation, and this is a common use in women who do not have diabetes.
- Alongside insulin in type 2 diabetes, reducing the insulin dose needed.
- Preventing weight gain caused by some antipsychotic medicines, in specific circumstances.
It manages diabetes rather than curing it, and it works alongside — not instead of — diet, physical activity and weight management, which remain the foundation.
Blood sugar control protects against complications that develop silently over years: damage to the eyes, kidneys, nerves, heart and blood vessels. Like blood pressure treatment, metformin is often treating a future risk rather than a present symptom. Many people with type 2 diabetes feel entirely well, which makes it easy to under-appreciate.
How Metformin fits into this medicine category
| Group | Examples | How it works |
|---|---|---|
| Biguanides | Metformin | Reduces liver glucose output; improves insulin sensitivity |
| Sulfonylureas | Gliclazide, glimepiride | Stimulate insulin release; can cause hypos and weight gain |
| DPP-4 inhibitors | Sitagliptin, vildagliptin | Enhance the body’s own incretin response |
| GLP-1 agonists | Semaglutide (Ozempic) | Injected; lower glucose and reduce weight |
| SGLT2 inhibitors | Empagliflozin, dapagliflozin | Excrete glucose in urine; heart and kidney benefits |
| Insulin | Various | Replaces or supplements the body’s own insulin |
Metformin is almost always the starting point, with others added if needed. See the endocrine and diabetes medicines hub.
What to check before using Metformin
Take it with food, or immediately after a meal. This substantially reduces the stomach upset that is metformin’s main drawback.
Start low and increase slowly. Metformin is normally introduced gradually over weeks precisely to let the gut adjust. Starting at a full dose is the most reliable way to make it intolerable.
Know the “sick day rules”. This is the most important safety point. Temporarily stop metformin if you become dehydrated — with vomiting, diarrhoea, high fever, or an inability to drink normally — and restart once you are eating and drinking properly. Dehydration impairs kidney function, metformin accumulates, and that is the setting for lactic acidosis. Ask your doctor to confirm this advice for you.
Tell your doctor or pharmacist if you:
- Have kidney disease. This is the key check — metformin is cleared by the kidneys, the dose depends on kidney function, and it is stopped below a certain level. Expect periodic blood tests.
- Have liver disease, or heart failure.
- Drink alcohol heavily. Alcohol adds to the risk of lactic acidosis.
- Are having a scan with contrast dye, or surgery. Metformin is usually paused around these — tell the department that you take it.
- Have a severe infection, or any acute serious illness.
- Are pregnant, planning pregnancy or breastfeeding. Metformin is used in pregnancy in some circumstances — this is a discussion, not an automatic stop.
- Take insulin or a sulfonylurea, which together with metformin can cause hypos.
- Take diuretics, ACE inhibitors, ARBs or anti-inflammatories such as ibuprofen, all of which can affect kidney function.
- Take cimetidine, dolutegravir or corticosteroids, which raise blood sugar.
Possible cautions and when to ask a pharmacist or doctor
The most common effects are nausea, diarrhoea, abdominal discomfort, wind, a metallic taste and reduced appetite. These are usually worst when starting or after an increase, and typically settle within a few weeks. Taking it with food, increasing slowly, or switching to slow-release usually resolves them.
Long-term metformin can reduce vitamin B12 absorption. This is worth knowing because the resulting deficiency causes tiredness and nerve symptoms — tingling and numbness in the hands and feet — which are easily mistaken for diabetic nerve damage. B12 levels are often checked periodically with long-term use.
Seek medical attention urgently if you develop:
- Deep or rapid breathing, severe weakness, unusual muscle pain, marked drowsiness, abdominal pain, feeling very cold, or vomiting — possible lactic acidosis. This is rare but a medical emergency, and it occurs almost exclusively when kidney function is impaired or you are seriously unwell or dehydrated. Stop the medicine and get help.
- Persistent vomiting or diarrhoea, or an inability to keep fluids down.
- Yellowing of the eyes or skin.
- A rash, or swelling of the face or lips.
- Symptoms of low blood sugar — shaking, sweating, confusion, hunger, palpitations — particularly if you also take insulin or a sulfonylurea. Treat with fast-acting sugar and tell your doctor.
Do not stop metformin because your readings are good. Good control usually means the treatment is working.
Contact your doctor if stomach effects persist beyond a few weeks, if blood sugar is consistently high or low, if you are planning pregnancy, or before any procedure involving contrast dye or anaesthetic.
Related medicine guides
Frequently asked questions
What is metformin used for? Type 2 diabetes, as the standard first-line treatment. It is also used in prediabetes and polycystic ovary syndrome.
What is metformin 500 mg used for? The usual starting strength for type 2 diabetes, taken with food and increased gradually as tolerated.
What is metformin 850 mg used for? The same purpose at a higher strength, generally reached after starting lower.
Does metformin cause low blood sugar? Not usually on its own, because it does not force insulin release. Hypos become a risk when it is combined with insulin or a sulfonylurea.
Why does it upset my stomach? Gut effects are metformin’s main drawback. Taking it with food, increasing slowly, or switching to the slow-release form usually helps — tell your doctor rather than stopping.
Should I stop it if I am ill? Pause it temporarily if you are dehydrated from vomiting, diarrhoea or high fever, and restart once eating and drinking normally. Confirm this advice with your doctor.
Does metformin cause weight loss? It does not cause weight gain and often causes modest weight loss, which is one reason it is first choice.
Why might I need vitamin B12 checked? Long-term metformin can reduce B12 absorption, and deficiency causes tiredness and nerve symptoms that resemble diabetic nerve damage.